Optimized liquid-phase oxidation

Abstract

Disclosed is an optimized process and apparatus for more efficiently and economically carrying out the liquid-phase oxidation of an oxidizable compound. Such liquid-phase oxidation is carried out in a bubble column reactor that provides for a highly efficient reaction at relatively low temperatures. When the oxidized compound is para-xylene and the product from the oxidation reaction is crude terephthalic acid (CTA), such CTA product can be purified and separated by more economical techniques than could be employed if the CTA were formed by a conventional high-temperature oxidation process.

1. A process for producing a carboxylic acid, said process comprising: introducing an oxidant stream comprising molecular oxygen into a reaction zone of a bubble column reactor via a plurality of oxidant openings defined in a conduit, wherein the pressure drop associated with discharging said oxidant stream from said conduit is not more than about 0.3 megaPascals (MPa), wherein said reaction zone comprises a reaction medium having a maximum height (H); and wherein said bubble column reactor is not mechanically agitated; and wherein a majority of said molecular oxygen enters said reaction medium within about 0.25H of the bottom of said reaction medium comprising oxidizing para-xylene in a liquid phase of a multi-phase reaction medium contained in said reaction zone of said bubble column reactor to form crude terephthalic acid. 
     
     
       2. The process of  claim 1  wherein the pressure drop associated with discharging said oxidant stream from said conduit is not more than about 0.2 MPa. 
     
     
       3. The process of  claim 1  wherein the pressure of said oxidant stream in said conduit is in the range of from about 0.35 to about 1 MPa. 
     
     
       4. The process of  claim 1  wherein at least about 5 weight percent of said oxidant stream is discharged from said conduit in a downward direction. 
     
     
       5. The process of  claim 1  wherein at least about 15 weight percent of said oxidant stream is discharged from said conduit at a downward angle within 30 degrees of vertical. 
     
     
       6. The process of  claim 1  wherein less than about 75 weight percent of said oxidant stream is discharged from said conduit in an upward direction. 
     
     
       7. The process of  claim 1  wherein at least a portion of said oxidant stream is discharged from said conduit at a downward angle in the range of from about 15 to about 75 degrees from vertical. 
     
     
       8. The process of  claim 1  wherein said conduit comprises at least 10 of said oxidant openings. 
     
     
       9. The process of  claim 8  wherein substantially all of said oxidant openings have a minimum diameter in the range of from about 2 to about 300 millimeters. 
     
     
       10. The process of  claim 1  wherein the pressure drop associated with discharging said oxidant stream from said conduit is not more than about 0.1 MPa, wherein the pressure of said oxidant stream in said conduit is in the range of from about 0.45 to about 0.85 MPa. 
     
     
       11. The process of  claim 1  wherein at least about 20 weight percent of said oxidant stream is discharged from said conduit in a downward direction, wherein at least about 15 weight percent of said oxidant stream is discharged from said conduit a downward angle within 30 degrees of vertical, wherein less than about 50 weight percent of said oxidant stream is discharged from said conduit in an upward direction. 
     
     
       12. The process of  claim 10  wherein said conduit comprises in the range of from about 20 to about 200 of said oxidant openings, wherein substantially all of said oxidant openings have a minimum diameter in the range of from about 4 to about 120 millimeters. 
     
     
       13. The process of  claim 1  wherein the pressure drop associated with discharging said oxidant stream from said conduit is not more than 0.05 MPa, wherein the pressure of said oxidant stream in said conduit is in the range of from 0.5 to 0.7 MPa. 
     
     
       14. The process of  claim 1  wherein at least 75 weight percent of said oxidant stream is discharged from said conduit in a downward direction, wherein at least 40 weight percent of said oxidant stream is discharged from said conduit at a downward angle within 30 degrees of vertical, wherein less than 5 weight percent of said oxidant stream is discharged from said conduit in an upward direction. 
     
     
       15. The process of  claim 13  wherein said conduit comprises in the range of from 40 to 100 of said oxidant openings, wherein substantially all of said oxidant openings have a minimum diameter in the range of from 8 to 60 millimeters. 
     
     
       16. The process of  claim 1  wherein said process further comprises introducing a liquid stream into said conduit and discharging said liquid stream into said reaction zone via one or more of said oxidant openings. 
     
     
       17. The process of  claim 16  wherein said introduction and discharging of said liquid stream flushes solids out of said conduit through said oxidant openings. 
     
     
       18. The process of  claim 1  wherein said molecular oxygen enters said reaction zone in a manner such that when said reaction zone is theoretically partitioned into 4 vertical quadrants of equal volume by a pair of intersecting vertical planes, not more than about 80 weight percent of said molecular oxygen enters said reaction zone in a single one of said vertical quadrants. 
     
     
       19. The process of  claim 18  wherein not more than about 60 weight percent of said molecular oxygen enters said reaction zone in a single one of said vertical quadrants. 
     
     
       20. The process of  claim 19  wherein at least about 25 weight percent of said molecular oxygen enters said reaction zone in a preferred feed zone, wherein said preferred feed zone is defined by a theoretical upright annulus having an outer diameter of 0.9D and an inner diameter of 0.2D, wherein said upright annulus is substantially centered in said reaction zone and said reaction zone has a maximum diameter (D). 
     
     
       21. The process of  claim 20  wherein at least about 50 weight percent of said molecular oxygen enters said reaction zone in said preferred feed zone. 
     
     
       22. The process of  claim 1  wherein at least a portion of said reaction zone is defined by one or more upright sidewalls of said reactor, wherein at least about 25 weight percent of said molecular oxygen enters said reaction zone at one or more locations spaced inwardly at least 0.05D from said upright sidewalls, wherein said reaction zone has a maximum diameter (D). 
     
     
       23. The process of  claim 22  wherein at least about 50 weight percent of said molecular oxygen enters said reaction zone at one or more locations spaced inwardly at least 0.05D from said upright sidewalls. 
     
     
       24. The process of  claim 1  wherein said oxidizing is carried out in a manner such that when the entire volume of said reaction medium is theoretically partitioned into 2,000 discrete horizontal slices of equal volume, less than 6 of said horizontal slices have a gas hold-up less than 0.1 on a time-averaged and volume-averaged basis. 
     
     
       25. The process of  claim 24  wherein the entire volume of said reaction medium has a gas hold-up of at least about 0.4 on a time-averaged and volume-averaged basis. 
     
     
       26. The process of  claim 1  wherein said reaction medium has an H:W ratio of at least about 3:1. 
     
     
       27. The process of  claim 26  wherein said H:W ratio is in the range of from about 8:1 to about 20:1. 
     
     
       28. The process of  claim 26  wherein a majority of said molecular oxygen enters said reaction zone within about 0.20W of the bottom of said reaction zone. 
     
     
       29. The process of  claim 26  wherein a majority of said molecular oxygen enters said reaction zone within 0.15W and 0.015H of the bottom of said reaction zone. 
     
     
       30. The process of  claim 26  wherein said process further comprises introducing a predominately liquid-phase feed stream comprising said oxidizable compound into said reaction zone, wherein at least about 30 weight percent of said oxidizable compound enters said reaction zone within about 1.5W of the lowest location where said molecular oxygen enters said reaction zone. 
     
     
       31. The process of  claim 30  wherein said feed stream is introduced into said reaction zone via a plurality of feed openings, wherein at least two of said feed openings are vertically spaced from one another by at least about 0.5W. 
     
     
       32. The process of  claim 1  wherein said reaction medium is a three-phase reaction medium. 
     
     
       33. The process of  claim 1  wherein said oxidizing causes the formation of solids in said reaction medium. 
     
     
       34. The process of  claim 1  wherein said oxidizing causes at least about 10 weight percent of said oxidizable compound to form solids in said reaction medium. 
     
     
       35. The process of  claim 1  wherein said reaction medium comprises in the range of from about 5 to about 40 weight percent solids on a time-averaged and volume-averaged basis. 
     
     
       36. The process of  claim 1  wherein said oxidizing is carried out in the presence of a catalyst system comprising cobalt. 
     
     
       37. The process of  claim 36  wherein said catalyst system further comprises bromine and manganese. 
     
     
       38. The process of  claim 1  wherein said process further comprises oxidizing para-xylene in a liquid phase of a multi-phase reaction medium disposed in said reaction zone to thereby form crude terephthalic acid, wherein said process further comprises oxidizing at least a portion of said crude terephthalic acid in a secondary oxidation reactor to thereby form purer terephthalic acid. 
     
     
       39. The process of  claim 38  wherein the pressure drop associated with discharging said oxidant stream from said conduit is not more than about 0.2 MPa, wherein the pressure of said oxidant stream in said conduit is in the range of from about 0.35 to about 1 MPa. 
     
     
       40. The process of  claim 38  wherein at least about 5 weight percent of said oxidant stream is discharged from said conduit in a downward direction. 
     
     
       41. The process of  claim 38  wherein at least about 15 weight percent of said oxidant stream is discharged from said conduit at a downward angle within 30 degrees of vertical. 
     
     
       42. The process of  claim 38  wherein less than about 75 weight percent of said oxidant stream is discharged from said conduit in an upward direction. 
     
     
       43. The process of  claim 38  wherein said conduit comprises at least 10 of said oxidant openings, wherein substantially all of said oxidant openings have a minimum diameter in the range of from about 2 to about 300 millimeters. 
     
     
       44. The process of  claim 38  wherein said oxidizing in said secondary oxidation reactor decreases the average concentration of 4-CBA in said crude terephthalic acid by at least about 200 ppmw to thereby form said purer terephthalic acid, wherein said crude terephthalic acid has an average 4-CBA concentration of at least about 400 ppmw and said purer terephthalic acid has an average 4-CBA concentration of less than about 400 ppmw. 
     
     
       45. The process of  claim 38  wherein said oxidizing in said secondary oxidation reactor decreases the average concentration of 4-CBA in said crude terephthalic acid by at least about 400 ppmw to thereby form said purer terephthalic acid, wherein said crude terephthalic acid has an average 4-CBA concentration of at least about 800 ppmw and said purer terephthalic acid has an average 4-CBA concentration of less than about 250 ppmw. 
     
     
       46. The process of  claim 38  wherein said oxidizing in said secondary oxidation reactor is carried out at an average temperature at least about 10° C. greater than the average temperature of said oxidizing in said bubble column reactor, wherein said oxidizing in said bubble column reactor is carried out at an average temperature in the range of from about 125 to about 200° C., wherein said oxidizing in said secondary oxidation reactor is carried out at an average temperature in the range of from about 160 to about 240° C. 
     
     
       47. The process of  claim 38  wherein said oxidizing in said secondary oxidation reactor is carried out at an average temperature in the range of from about 20 to about 80° C. greater than the average temperature of said oxidizing in said bubble column reactor, wherein said oxidizing in said bubble column reactor is carried out at an average temperature in the range of from about 140 to about 180° C., wherein said oxidizing in said secondary oxidation reactor is carried out at an average temperature in the range of from about 180 to about 220° C. 
     
     
       48. The process of  claim 38  wherein a substantial portion of said crude terephthalic acid exists as solid crude terephthalic acid particles having an average BET surface area of at least about 0.6 meters squared per gram. 
     
     
       49. The process of  claim 48  wherein said solid crude terephthalic acid particles have an average particle size in the range of from about 20 to about 150 microns. 
     
     
       50. The process of  claim 49  wherein a substantial portion of said solid crude terephthalic acid particles are formed of a plurality of agglomerated sub-particles having an average particle size in the range of from about 0.5 to about 30 microns. 
     
     
       51. The process of  claim 50  wherein said solid crude terephthalic acid particles have an average particle size in the range of from about 30 to about 120 microns, wherein said sub-particles have an average particle size in the range of from about 1 to about 15 microns. 
     
     
       52. The process of  claim 38  wherein said process further comprises recovering an initial slurry comprising a mother liquor and said crude terephthalic acid from said bubble column reactor, wherein said process further comprises replacing at least 50 weight percent of said mother liquor in said initial slurry with a replacement solvent to thereby provide a solvent-exchanged slurry comprising said replacement solvent and said crude terephthalic acid, wherein said process further comprises introducing said solvent-exchanged slurry into said secondary oxidation reactor.